Generally, in a flash apparatuses, it is desirable to control the intensity and the colour temperature of the light emitted from the flash tube during a flash as well as the total amount of light emitted from the flash tube.
A flash apparatus typically comprises a generator comprising at least one capacitor configured to apply a voltage to the flash tube. The flash tube comprises a fluid, for example a gas. The fluid in the flash tube is in a normal state an insulator. When a flash is desired, a triggering of the flash tube is performed. There are several methods of triggering the flash tube. For example, a triggering electric current may be supplied to a wire arranged in or around the flash tube. The triggering electric current ionizes parts of the fluid in the flash tube, which causes the fluid to start conducting electric current between the electrodes of the flash tube. The electrical contact between the electrodes of the flash tube allows the at least one capacitor to discharge, causing a pulse of electric current to be led through the fluid in the flash tube. This pulse of electric current causes the fluid in the flash tube to emit light. The emitted light has a certain intensity and a certain colour temperature at each point in time during the flash. In addition, a total amount of light is emitted during the flash. The characteristics of the emitted light is dependent on the energy supplied to the flash tube.
The colour temperature of the total amount of light emitted from the flash tube during a flash is an important factor when the flash is used for photographic purposes. In the beginning of the flash, that is, in the beginning of the discharge of the capacitor, normally, light with a higher colour temperature is emitted from the flash tube. At the end of the flash, that is, at the end of the discharge of the capacitor, light with a lower colour temperature is emitted from the flash tube. It is desirable to be able to control the colour temperature of the flash.
In addition, or alternatively, it is desirable to control the intensity of light emitted during a flash and/or the total amount of light emitted from the flash tube during a flash.
One method of controlling the total amount of light and the intensity of the light emitted from a flash tube is to adjust the charging voltage of the capacitors of the generator. The lower the voltage applied to the flash tube, the lower the total amount of light and the intensity of the light emitted during the flash. However, the adjustment of the charging voltage also affects the colour temperature of the emitted light.
Another method of controlling the characteristics of the light emitted from the flash tube during a flash is to cut the electrical current supplied to the flash tube at a certain time before the capacitors have discharged completely. By cutting the current to the flash tube at a certain time, the flash tube will stop emitting light and hence, the characteristics of the light emitted from the flash tube may be controlled. With this method, for example a change in colour temperature of the light emitted from a flash tube during a flash caused by a change in charging voltage of the capacitors may be compensated for by cutting the electrical current to the flash tube at a certain point in time.
Another way of controlling the light emitted from a flash tube is to control the current from the capacitor to the flash tube as the capacitor discharges. This may be performed by switching a switch in the drive circuit on and off, thereby interrupting the current in the drive circuit at certain intervals. By varying the current supplied to the flash tube, the intensity, the colour temperature of the emitted light and the total amount of emitted light can be controlled.
WO2014126528 describes a drive circuit for a flash tube wherein a switch is used in order to control the current in the drive circuit.
One problem associated with this technique of interrupting the power supply to the flash tube during the discharge of the capacitor in the drive circuit is that due to the large amount of energy transmitted to the flash tube during a flash the components of the drive circuit for the flash tube are exposed to a large strain which may cause damage on the components and thereby failure of the flash. Another problem with the prior art is that the switch has a maximum switching rate. Hence, the current in the drive circuit may only be turned on and off a certain amount of times during one second. This switching rate affects how accurate the control of the current supplied to the flash tube will be.
The above mentioned problems occur when controlling the power supplied to a flash tube in all fields of technology.
US20040085026, which concerns pulsed flash lamps used in a variety of applications such a cosmetology, describes one method of protecting the components of a drive circuit of a pulsed flash lamps.
There is a need and desire for a method to control the power supply to a flash tube during a flash and at the same time protect the components of a drive circuit.